The operation of discharge lamps requires ballasts, for example in form of iron-cored reactors, as is widely known. Additionally required are ignition devices which generate high-voltage pulses to switch the lamps on and by means of which the ionization of the gas mixture existing in the lamps is initiated.
Ignition devices of discharge lamps usually work in accordance with the so-called superposition principle similar to a tesla transformer. Known ignition devices are comprised of an ignition pulse generator to generate the required voltage pulses. The ignition pulse generator is comprised of an ignition circuit which consists of a surge capacitor, the primary winding of a superimposed transformer and a switching element. The surge capacitor must be charged with high voltage to achieve the required voltage of ignition pulses. With prior art ignition devices, this purpose is served by a high-voltage generator of the ignition pulse generator. The high-voltage generator usually works by the aid of a transformer to generate the high voltage needed for charging from the available line voltage. During the ignition procedure, the charged surge capacitor is periodically discharged through the primary winding of the superimposed transformer by switching the switching element periodically on and off. In the switched-on status, the surge capacitor and the primary winding of the superimposed transformer develop a high-frequency resonant circuit. The high-frequency oscillations are transformed-up in the secondary winding of the superimposed transformer connected with the lamp and are available to the lamp as ignition voltage.
Frequently used as switching elements with prior art ignition devices are spark gaps. The spark gap ignites when the surge capacitor has been charged to a certain starting (turn-on) voltage. The spark gap continues to be conductive until the surge capacitor has been discharged via the primary winding of the superimposed transformer to a certain turn-off voltage.
Owing to the high electric currents which flow in the ignition circuit when discharging the surge capacitor, the gas ionized in the spark gap is heated-up to such an extent that de-ionization is impeded. For this reason, it takes an undesirably long time before the spark gap turns-off. The relatively long de-ionization time takes the effect that ignition pulses can only be generated with comparably low frequencies. But it is especially with ignition devices which are to be suitable for hot ignition of high-pressure discharge lamps that it is required to generate ignition pulses with a high voltage (20 to 60 kV) with a particularly high frequency, because as a rule more than 1000 ignition pulses per second are necessary during an ignition time of several seconds in order to ensure a reliable re-ignition of high-pressure discharge lamps operating at rated load.
Known from DE 86 16 255 U1 is a switching arrangement for an ignition device of a discharge lamp. With this switching arrangement, a multiple-stage spark gap is implemented which is composed of two or more individual spark gaps. As compared with single-stage spark gaps, a division into several spark gaps has the advantage that the de-ionization time is noticeably reduced. Among other reasons, this is attributable to the improved cooling obtained with a multiple-stage arrangement. The prior art switching arrangement is particularly suitable for use in an ignition circuit of a high-pressure discharge lamp. The multiple-stage spark gap allows for generating up to 4000 ignition pulses per second.
However, the switching arrangement known from prior art in technology has the drawback that the spark gap is of a comparably costly and expensive design and construction. With the prior art arrangement, the electrodes of the spark gap constitute massive copper bodies which are concentrically arranged, one behind the other in axial direction, in an insulating material tube. The gas discharge slots between the individual electrodes are filled with a special gas mixture which is partly comprised of hydrogen. The front faces of the electrodes are provided with an activation layer made of sodium silicate and another metal component. On account of the special materials of the electrodes and because of the gas filling of the entire arrangement, the prior art spark gap is extremely costly in production. The ignition devices for discharge lamps equipped therewith are accordingly expensive.